New webhook trigger for receiving Pull Request review requests (#24481)

close https://github.com/go-gitea/gitea/issues/16321

Provided a webhook trigger for requesting someone to review the Pull
Request.

Some modifications have been made to the returned `PullRequestPayload`
based on the GitHub webhook settings, including:
- add a description of the current reviewer object as
`RequestedReviewer` .
- setting the action to either **review_requested** or
**review_request_removed** based on the operation.
- adding the `RequestedReviewers` field to the issues_model.PullRequest.
This field will be loaded into the PullRequest through
`LoadRequestedReviewers()` when `ToAPIPullRequest` is called.

After the Pull Request is merged, I will supplement the relevant
documentation.

templates/swagger/v1_json.tmpl

@@ -19934,6 +19934,13 @@
           "type": "string",
           "x-go-name": "PatchURL"
         },
+        "requested_reviewers": {
+          "type": "array",
+          "items": {
+            "$ref": "#/definitions/User"
+          },
+          "x-go-name": "RequestedReviewers"
+        },
         "state": {
           "$ref": "#/definitions/StateType"
         },